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LuSadagopanKrishnamachariGoel_Infocom05 - 1 Delay Efcient...

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1 Delay Efficient Sleep Scheduling in Wireless Sensor Networks Gang Lu * , Narayanan Sadagopan , Bhaskar Krishnamachari *† , Ashish Goel * Department of Electrical Engineering Department of Computer Science University of Southern California, Los Angeles, 90089 Department of Management Science and Engineering Stanford University, Stanford CA 94305-4026 { ganglu, narayans, bkrishna } @usc.edu,[email protected] Abstract — Medium access techniques for wireless sensor networks raise the important question of providing periodic energy-efficient radio sleep cycles while minimizing the end- to-end communication delays. This study aims to minimize the communication latency given that each sensor has a duty cycling requirement of being awake for only 1 k time slots on an average. As a first step we consider the single wake-up schedule case, where each sensor can choose exactly one of the k slots to wake up. We formulate a novel graph-theoretical abstraction of this problem in the general setting of a low- traffic wireless sensor network with arbitrary communication flows and prove that minimizing the end-to-end communication delays is in general NP-hard. However, we are able to derive and analyze optimal solutions for two special cases: tree topologies and ring topologies. Several heuristics for arbitrary topologies are proposed and evaluated by simulations. Our simulations suggest that distributed heuristics may perform poorly because of the global nature of the constraints involved. We also show that by carefully choosing multiple wake-up slots for each sensor significant delay savings can be obtained over the single wake-up schedule case while maintaining the same duty cycling. Using this technique, we propose algorithms that offer a desirable bound of d + O ( k ) on the delay for specialized topologies like the tree and grid and a weaker guarantee of O (( d + k )log n ) for arbitrary graphs, where d is the shortest path between 2 nodes in the underlying topology and n is the total number of nodes. Keywords: Mathematical Optimization, Graph Theory, Combinatorics, System Design I. I NTRODUCTION Wireless sensor networks (WSN) are expected to operate for months if not years on small inexpensive batteries with limited lifetimes. Therefore energy efficiency is typically the primary goal in these networks. Previous works have identified idle listening of the radio as a major source of energy wastage (e.g. [1], [3], [4], [5], [6], [7], [8]). Measurements on existing sensor device radios show that idle listening consumes nearly the same power as receiving. In sensor network applications where the traffic load is very light most of the time, it is therefore desirable to turn off the radio when a node does not participate in any data delivery.
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